Diethylphosphite (DEP) and its derivative exhibited thermally latent properties for epoxy curing reactions through the formation of phosphonic acid as an active species from a reaction of ethanol elimination, which was observed with H-1 NMR and pyrolysis gas chromatography/mass spectrometry measurements. The thermally latent properties and curing reaction kinetics of the curing reaction of DEPs with diglycidyl ether of bisphenol A were studied with differential scanning calorimetry. The cured epoxy resins possessed a phosphorous element coming from the DEP derivatives, exhibiting improved flame retardancy.